The present invention relates to a pilot operated directional control valve having a position detecting function, improved by permitting the detection of operating positions of a valve member such as a spool, through the use of light.
The directional control valve capable of detecting the changeover position of a spool utilizing a magnet is well known as disclosed in, for example, Japanese Unexamined Utility Model Publication No. 2-66784 (Japanese Utility Model Application No. 63-144027 on microfilm). This directional control valve detects the changeover position of the spool by mounting a magnet on the spool, mounting a magnetic sensor on a casing, and detecting the magnet moving together with the spool by means of the magnetic sensor.
However, the conventional directional control valve described in the above-mentioned known document adopts the method wherein a magnet is used as an object to be detected, and wherein the magnetism from this magnet is detected by a magnetic sensor, and therefore, the conventional directional control valve is susceptible to an external magnetic field. For example, under the circumstances where a welder in which a large current flows or where a large-sized motor is used, the magnetic sensor can cause a malfunction due to an external magnetic field generated by electrical current. This makes it difficult to maintain a given detecting accuracy, and has created a problem in reliability.
On the other hand, the above-mentioned known document includes a proposition of detecting the changeover position of a spool using an optical sensor. In accordance with this method, positions of the spool can be detected without being affected by external magnetic field, but this document does not show any specific construction for substantiating this proposition.
The main technical problem of the present invention is to provide a simply constructed pilot operated directional control valve having a position detecting function, improved by permitting the detection of operating positions of a valve member such as a spool, using an optical sensor.
The subordinate technical problem of the present invention is to increase the ease of maintenance by disposing the optical sensor at a predetermined position on a casing, in the above-mentioned directional control valve.
In order to solve the above-described problems, in accordance with the present invention, there is provided a pilot operated directional control valve having a position detecting function, comprising a plurality of ports; a valve hole to which each of these ports is opened; a casing having the ports and the valve hole; a valve member slidably received in the valve hole for changing over flow passages; a piston chamber formed on at least one end side of the valve member; a piston which is slidably received in the piston chamber, and which operates by the action of pilot fluid pressure to change over the valve member; a pilot valve for supplying the piston with a pilot fluid; a through hole formed at a portion of the casing so that the object to be detected on the piston can be seen through therethrough; an optical sensor disposed so as to be able to project and receive light through the through hole for detecting operating positions of the piston by the light reflected from the piston.
In the directional control valve having the above-described described features, when the piston is supplied with a pilot fluid by the pilot valve, the piston operates by the action of the pilot fluid pressure and changes over the valve member. At this time, through the through hole of the casing, the projection of light to the piston and the reception of the reflected light is performed by the optical sensor, and thereby operating positions of the piston, or that of the valve member is detected in response to the change in quantity of received light.
Thus, in accordance with the present invention, it is possible to execute the detection of positions of a valve member using a optical sensor without being affected by external magnetic field. In addition, since the present directional control valve is constituted so as not to directly detect the movement of the valve member but to indirectly detect positions of the valve member by detecting the piston, the structure of a detecting mechanism becomes simpler. If the present directional control valve were constituted to directly detect a valve member, the through hole would have to be formed at a place from which the valve member inside a valve hole can be seen through therethrough so that light can be projected and received relative to the valve member. However, since such a place corresponds to a portion directly facing a passage of a hydraulic fluid, it is necessary to form the through hole and install the optical sensor so as not to cause leakage of fluid. This makes the structure complicated and makes the machining difficult. In contrast, since the present invention is constituted so as to form a through hole at a portion of the casing from which a portion of the piston can be seen through the through hole, and to project and receive light relative to the piston through the through hole, it is possible to install the detecting mechanism at a place independent of the passage of a hydraulic fluid. This eliminates the necessity for structural considerations of preventing the hydraulic fluid from leakage.
In accordance with a specific embodiment of the present invention, the above-described directional control valve is constituted so as to comprise breathing chambers which are opened to the outside and which are each disposed between the valve member and the piston; an object to be detected which protrudes parallel with the axis of the valve member and which is situated on the end face of the piston, adjacent to a breathing chamber; a recess in which the object to be detected is fit and which is disposed in the wall surface of the breathing chamber, in the casing, and the through hole formed at a position facing the recess, whereby the object to be detected is detectable in the recess.
Thereby, even if the body of the piston can not be directly detected because the installation site for an optical sensor can not be secured around the piston chamber, the piston can reliably be detected via the object to be detected. In addition, even if the optical sensor is made to directly face the object to be detected by making the through hole opened to the recess, there is no need to worry about a hydraulic fluid or a pilot fluid leaking through this through hole, since the recess is formed within the breathing chamber.
In accordance with a preferred embodiment of the present invention, the object to be detected is formed into annular shape having the same diameter as that of the piston, and the recess is formed into annular shape having the same diameter as that of the piston chamber.
In accordance with another specific embodiment of the present invention, there is provided a directional control valve having a constitution wherein the through hole is formed at a position opposite to an end face of the piston, on the central axis of the piston, wherein the optical sensor is disposed so as to be able to project and receive light relative to the end face of the piston, and wherein operating positions of the piston is detected based on the change in quantity of the received light caused by the change in the distance due to the displacement of the piston.
This features can be suitably applied, in a double-pilot operated directional control valve having two pistons and the two pilot valves, to the case where the two pilot valves are concentratedly provided on one side of the casing. By installing the optical sensor onto the other side where no pilot valve of the casing is provided, the installation of the optical sensor becomes simpler and also the maintenance thereof becomes much easier.